1. Field of the Invention
This invention relates to a method of preparing maleic anhydride; more particularly, in the improvment of the catalysts utilized for preparing maleic anhydride.
2. Description of the Prior Art
In preparing maleic anhydride by the catalytic oxidation of unsaturated hydrocarbons containing 4 or more carbon atoms in each molecule, phosphorus-vanadium-oxygen type catalysts are typically used. Although catalysts of this type can produce maleic anhydride in high yields when the concentration of the starting material unsaturated hydrocarbon is low, they are not fully satisfactory because the yield of the maleic anhydride is decreased as the concentration of the raw material is increased. A similar disadvantage is also encountered when catalysts consisting of oxides of molybdenum or antimony are used. An example of such a catalyst consisting of molybdenum containing oxides is disclosed in Japanese Patent Publication No. 685/1972. The catalyst disclosed therein has the general composition: EQU V.sub.a P.sub.b Mo.sub.c Ni.sub.d Fe.sub.e Bi.sub.f X.sub.g O.sub.h
where X represents magnesium and/or cobalt, and a, b, c, d, e, f, g and h represent the number of atoms of vanadium, phosphorus, molybdenum, nickel, iron, bismuth, magnesium and/or coablt, respectively. When these molybdenum or antimony containing oxides are used as catalysts, it is necessary to decrease the concentration of the starting material unsaturated hydrocarbons to an extremely low value; to use pure oxygen as an oxidizing agent; or to supply a large volume of steam into the reaction system. For this reason, it is difficult to use these catalysts on an industrial scale and a need exists for an industrially suitable catalyst.